The present invention relates to a fuel composition which combusts instantaneously, to an instantaneous combustion method which uses the said composition and to a plant for carrying out the said method.
More particularly, the present invention relates to a fuel composition which combusts instantaneously, comprising a fossil fuel and a non-fossil solid fuel (NFSF) chosen from the group comprising urban solid waste (USW), elastomeric and non-elastomeric polymer materials, and mixtures thereof.
Many methods have up to now been proposed for destroying and/or recovering, at least partly, used plastic wrapping materials, tyres and/or urban solid waste.
However, the problem is very complex since the nature and composition of urban solid waste varies from one place to another and from day to day, and on account of the presence of putrefiable organic waste.
A number of the methods proposed to date envisage the removal of the putrefiable organic phase from the solid products, which can then be dried, ground and agglomerated. Generally, the agglomeration is carried but by compression into granules or tiles which are then destroyed by combustion in ovens with a moving grate or with a rotating drum in which they remain in the high-temperature zone in which the combustion takes place, for a period which is sufficient to obtain virtually complete combustion of the combustible mass. This duration, which can readily be controlled and calculated by dividing the length of the path travelled by the said mass in the said combustion zone by the throughput speed, is at least 1 minute, preferably at least 3 minutes, generally 5 minutes or more.
In the Applicant""s view, a drawback of these methods is that these granules or tiles cannot be used to feed instantaneous-combustion burners. Yet another drawback is that the preparation of granules and tiles involves a number of stages, such as a compression stage, which involve a certain expenditure of energy and thus increase the costs, making the abovementioned known methods uneconomical.
A fuel composition has now been found which is produced while avoiding the formation of agglomerates and which makes it possible to economize on large amounts of fossil fuels in the production of energy.
In its first aspect, the present invention relates to a fuel composition which combusts instantaneously, comprising from 40 to 95% by weight of an instantaneously combustible fossil fuel and from 60 to 5% by weight of a non-fossil solid fuel (NFSF) chosen from the group comprising urban solid waste, elastomeric and non-elastomeric polymer materials and mixtures thereof, this fuel being suitably treated so as to be instantaneously combustible.
Preferably, the amount of the said instantaneously combustible fossil fuel is between 50 and 90% by weight and that of the NFSF is between 50 and 10% by weight. Even more preferably, the amount of the said instantaneously combustible fossil fuel is between 60 and 80% by weight and that of the NFSF is between 40 and 20% by weight.
Throughout the present description and the claims, the term xe2x80x9cinstantaneous combustionxe2x80x9d is used to indicate a combustion which includes the combustion of at least 90% by weight of the fuel material fed into a burner in less than 10 seconds, preferably less than 5 seconds, even more preferably less than 3 seconds.
Typical examples of instantaneously combustible fossil fuels are methane, fuel oil, which may be in emulsion form, and fossil coal dust, which may be in the form of an aqueous suspension.
Typical examples of processing to which urban solid waste is subjected before it can be used as instantaneously combustible NFSF comprise, in a non-limiting manner, removal of any putrefiable organic compounds and of any metals, drying, grinding and screening. The urban solid waste thus processed will be referred to hereinbelow as USW. In the case of elastomeric and non-elastomeric polymer materials, suitable processing comprises, in a non-limiting manner, removal of any metals, drying, grinding and screening.
Preferred examples of fuel compositions which are instantaneously combustible according to the present invention are:
coal dust+USW,
coal dust+non-elastomeric polymer material,
coal dust+USW+non-elastomeric polymer material,
coal dust+USW+elastomeric polymer material,
coal dust+non-elastomeric polymer material+elastomeric polymer material,
coal dust+USW+non-elastomeric polymer material+elastomeric polymer material,
methane gas+USW,
methane gas+non-elastomeric polymer material,
methane gas+USW+non-elastomeric polymer material,
methane gas+USW+elastomeric polymer material,
methane gas+non-elastomeric polymer material+elastomeric polymer material,
methane gas+USW+non-elastomeric polymer material+elastomeric polymer material.
A typical example of a suitable non-fossil solid fuel composition comprising USW and elastomeric and non-elastomeric polymer materials is described in the Applicant""s Italian patent application No. MI97A 02890 of Dec. 30, 1997.
Typically, the non-fossil solid fuel composition of the abovementioned Italian patent application No. MI97A 02890 comprises a first fraction consisting of USW, a second fraction consisting of elastomeric material and a third fraction consisting of non-elastomeric polymer material. Preferably, the composition has an apparent density is equal to or less than 0.6 g/cm3 and the amount of each of the three fractions is preselected as a function of the desired calorific power. Typically the value of the apparent density is of from 0.2 to 0.6 g/cm3 and, preferably, of from 0.3 to 0.5.
One of the surprising properties of NFSF, observed during the experimental tests described later, is that it behaves like an ordinary instantaneously combustible solid fuel of average calorific power. It has also been observed that efficient grinding and/or shredding of its components to preset values makes it possible to reduce the content of non-combusted materials both in the heavy ash and in the fly ash, bringing this content to levels similar to those of the fossil fuel.
In general, the process for preparing the solid fuel composition of the abovementioned Italian patent application No. MI97A 02890 comprises the following stages:
a) an urban solid waste is processed in order to remove
a1) the putrefiable organic fraction by screening,
a2) ferrous materials, using a magnetic separator,
a3) aluminium, using an eddy-current magnet,
b) the fraction of urban solid waste thus obtained is shredded,
c) a waste material consisting of non-elastomeric polymer material is shredded,
d) the abovementioned fractions of urban solid waste and of non-elastomeric polymer material shredded are dried to a moisture content of less than or equal to 10%,
e) the abovementioned fractions are ground,
f) an elastomeric material is ground and stripped of any ferrous material associated therewith,
g) the abovementioned ground fractions are mixed together in a weight ratio which is predetermined as a function of the desired calorific power.
Examples of suitable elastomeric materials are used tyres. Examples of suitable non-elastomeric polymer materials are packaging and wrapping made of thermoplastic and/or thermosetting materials. Typical examples of thermoplastic materials mainly used in the production of wrapping material are: PE, LDPE, HDPE, PP, PET, polystyrene, ethylene/C4-C12 xcex1-olefin co-polymers, vinyl polymers and copolymers, and the like.
In its second aspect, the present invention relates to a fuel composition which combusts instantaneously, comprising from 40 to 95% by weight of an instantaneously combusting fossil fuel and from 60 to 5% by weight of particles smaller than 1 mesh (15 mm) in size of a fuel material chosen from the group comprising USW, elastomeric and non-elastomeric polymer materials, and mixtures thereof.
Preferably, at least 90% by weight of the abovementioned particles are smaller than 2 mesh (7.5 mm) in size. Even more preferably, at least 50% by weight of the abovementioned particles are smaller than 4 mesh (3.75 mm) in size.
Within the said composition, the granules of elastomeric polymer, if present, are less than 5 mm in size.
The relatively large size of the NFSF particles constitutes an entirely unexpected characteristic of the present invention. The reason for this is that traditional instantaneously combusting fuel materials consist of gaseous and liquid fossil fuels. However, a solid fossil fuel, such as coal, is suitable for use as an instantaneously combusting fuel material only after it has been very finely ground until 99% of the particles thereof are less than 100 mesh (0.15 mm) in size and 70% of these particles are less than 200 mesh (0.075 mm) in size. Thus, it was not foreseeable that NFSF would be suitable for instantaneous combustion even though its particles are at least 50 times as large as those of coal dust.
Moreover, it was considered that the essential element had to be the size of the elastomeric polymer granules and that they had to be similar in size to that of granules of coal dust. However, it was found, surprisingly, that it is sufficient for the elastomeric polymer granules to be less than 5 mm in size.
Furthermore, in the case of the composition which constitutes the second aspect of the invention, the amount of the said instantaneously combusting fossil fuel is preferably between 50 and 90% by weight and that of the said fuel material is between 50 and 10% by weight. Even more preferably, the amount of the said instantaneously combusting fossil fuel is between. 60 and 80% by weight and that of the said fuel material is between 40 and 20% by weight.
A typical example of a suitable non-fossil solid fuel composition comprising USW and elastomeric and non-elastomeric polymer materials and of a method for producing it is described in the abovemetioned Italian patent application No. MI97A 02890.
In a preferred embodiment of the present invention, the fuel material chosen from the group comprising USW, elastomeric and non-elastomeric polymer materials and mixtures thereof consists of a mixture comprising from 40 to 80% by weight of dry urban solid waste, from 10 to 50% by weight of elastomeric material and from 10 to 50% by weight of non-elastomeric polymer material. Even more preferably, this mixture comprises from 60 to 80% by weight of dry urban solid waste, 10 to 30% by weight of elastomeric material and from 10 to 30% by weight of non-elastomeric polymer material.
In its third aspect, the present invention relates to a combustion method in which the flame of a burner of an instantaneous-combustion boiler is fed with a flow of instantaneously combusting fuel material comprising from 40 to 95% by weight of an instantaneously combusting fossil fuel and from 60 to 5% by weight of a fuel material chosen from the group comprising USW, elastomeric and non-elastomeric polymer materials and mixtures thereof, which has been suitably treated so as to be instantaneously combustible.
In a preferred embodiment, the said fossil fuel is fed through a burner of known type, while the instantaneously combusting non-fossil fuel is fed into the region of the boiler known as the xe2x80x9cfire areaxe2x80x9d, i.e. the region of the boiler in which the temperature is above 1400xc2x0 C. Generally, the temperature of the fire area is between 1500 and 2000xc2x0 C.
Typically, the said fuel material is instantaneously combustible when it consists of particles less than 1 mesh (15 mm) in size. Preferably, at least 90% by weight of the abovementioned particles are less than 2 mesh (7.5 mm) in size. Even more preferably, at least 50% by weight of the abovementioned particles are less than 4 mesh (3.75 mm) in size. The elastic polymer, if present, is preferably in the form of granules less than 5 mm in size.
Preferably, the flow of instantaneously combusting fuel material used in the combustion method according to the present invention comprises from 50 to 80% by weight of instantaneously combusting fossil fuel and from 50 to 20% by weight of fuel material chosen from the group comprising USW, elastomeric and non-elastomeric polymer materials and mixtures thereof. Even more preferably, the amount of the said instantaneously combusting fossil fuel is between 60% and 80% by weight and that of the said fuel material is between 40 and 20% by weight.
The method of the present invention has proved to be particularly suitable for running power plants, i.e. plants generally dedicated to the production of steam for the production of electrical energy and/or for remote heating, and which produce an amount of steam greater than 40 thermal megawatts (tMW). Generally, the said power plants are considered small when they produce less than 50 tMW, medium-sized when they produce from 50 to 500 tMW and big when they produce more than 500 tMW.
In another embodiment thereof, the combustion method of the present invention comprises feeding an instantaneously combusting fuel material, comprising at least one fossil fuel and a non-fossil fuel chosen from the group comprising USW, elastomeric and non-elastomeric polymer materials and mixtures thereof, into a zone of a boiler which is at a temperature such that the level of non-combusted materials in the heavy ash is maintained at less than 50% by weight.
In another embodiment thereof, the combustion method of the present invention comprises feeding a boiler with an instantaneously combusting fuel material comprising at least one fossil fuel and a non-fossil fuel chosen from the group comprising USW, elastomeric and non-elastomeric polymer materials and mixtures thereof, in which the particle size of the said non-fossil fuel has been predetermined such that the level of the non-combusted materials in the heavy ash is maintained at less than 50% by weight.
In its fourth aspect, the present invention thus relates to a plant for the instantaneous combustion of at least one instantaneously combusting fossil fuel, comprising a boiler, at least one burner, a combustion zone and a system for feeding in at least one said instantaneously combusting fossil fuel, characterized in that it also comprises a device for feeding a non-fossil solid fuel (NFSF) into the said combustion zone.
According to two preferred embodiments of the present invention, the said NFSF supply device is of the mechanical or pneumatic type. In particular, the mechanical supply device is preferably an Archimedean screw device.